Process of making stable silica sols and resulting composition



Patented Dec. 4, 1951 PROCESS OF MAILING STABLE SILICA SOLS AND RESULTING GOD/[POSITION Joseph M. Rule, Cleveland, ()hio, assignor to E. I. du Pont de Nemours & Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application September 8,1950,

Serial No. 183,902

22 Claims. (01. 252313) This invention relates to novel silica sols and methods for making them. More particularly, the invention is concerned with sols which are stable, even when concentrated to upwards of 35% SiOz, by reason of the fact that they contain amorphous silica particles which are dense, non-agglomerated, spherical and to 130 millimicrons in diameter, and that'they are substantially salt-free and contain enough alkali metal hydroxide to give a silica: alkalimole ratio from 130:1 to 500:1. The invention is further directed to processes for producing such sols wherein a quantity of alkali metal hydroxide sufficientto produce the desired silica alkali ratio is added to a sol containing amorphous particles which are dense, non-agglomerated, spherical and 10 to 130 millimicrons in diameter, the sol being substantiallyjree of electrolytes.

In application Ser. No. 65,536, filed December 15, 1948, by Bechtold and Snyder and which issued as Patent No. 2,574,902 on November 13,. 1951, it has been shown that silica sols which are stable against gelation can be made byheating an aqueous sol of silica particles less than 10 millimicrons in diameter above 60 C. to form a heel, adding to said heel a silica sol containing particles less than 10 millimicrons in diameter, and continuing the addition and heating until at least five times as much silica has been added to the heel as was originally present. The sols thus formed are characterized by having a silica alkali ratio of from 60:1 to 130:1 and by being stable against gelation for extended periods. The sols are restricted in their usefulness by the fact that they are not very compatible with watersoluble organic liquids such as alcohols and ketones and by the further fact that when the sols are frozen the silica is precipitated in a form which isnot readily redispersible V In application Ser. No. 97,090, filed June 3, 1949, by Wolter, now abandoned, it is shown that a sol such as that described by Bechtold and Snyder may be deionized by subjecting the sol to suecessive contact with a cation exchanger and an anion exchanger. The sol is comparatively unstable aiter complete purification by this method,

particularly at higher concentrations-that is, at concentrations above about 80% SiOz. As shown in application Ser. No. 138,933 filed January 16, 1950, by Wolter, the so]. can be stabilized by adding a nitrogen base such as an organic amine or a quaternary ammonium hydroxide. For some uses the presence of small amounts of such bases is advantageous, as for instance, when a volatile base is desired; forother uses of the sol, howeventhe presence of a nitrogen basemay not be desirable, and in these situations no method for stabilizinga deionized sol has'hitherto been known. Furthermore, when it is attempted to concentrate sols stabilized with a volatile nitrogen base, the sols gel ratherquickly when the volatile base is removed. Q U

It is an object of this invention to provide aqueous silica sols which are compatible with water-soluble organic liquids. Another object 'is to provide aqueous silica sols which, after freezing in the presence of an alcohol or other organic liquid, are readily redispersible. Another object is to provide silica sols in which stability against gelling is achieved without the-necessity of having present a nitrogen base. Another object is to provide silica sols which are stabilized with a base of an alkali metal, the ratio of silica to alkali metal being inthe range from :1 to 500:1, and which contain more than 30% SiOz.

Another object is to provide processes for prosilica alkali mole ratio of from 130:1 to 500:1,

and are further accomplished by processes for producing such sols wherein a quantity of alkali metal hydroxide sufiicient to produce the desired silica alkali ratio is added to a sol containing amorphous silicaparticles which are dense, non

agglomerated, spherical and 10 to 130 millimicrons in diameter, the sol being substantially free of electrolytes.

The silica particlesin a sol of this invention are amorphous, giving no evidence of crystal linity as determined by conventional X-ray diffraction methods. That the particles are dense is indicated by measuring their surface area on an electron micrograph and comparing this value with the surface area as determined by nitrogen adsorption, a fairly close agreement indicating that the particles are substantially free of porespenetrable by nitrogen. The nonagglomerated'nature of the silica particles is shown by the fact that the sols have a relative viscosity of from 1.15 tov 1.55, as measured at 10 per cent S102 and at pH '10; agglomerated particles would give a higher'relative viscosity.

The spherical character of theparticles. and

their average size, is observable by means" of the electron microscope. A dilute sol, containing 0.25 per cent SiOz by weight, may for instance, be dried down and prepared for observation in the customary manner, and from the electron micrograph it will be observed that the particles are spherical. and non-agglomerated, the individual ultimate particles having an average size of from 10 to 130 millimicrons.

The salt-free nature of the sols is indicated by the fact that they have a specific conductance less than where R is the S102 alkali metal oxide mole ratio, the conductance being determined, at 28 C. and 10% by weight of S102, by conven-. tional methods. The mole ratio of S102 to alkali metal oxide in the sols is from 130:1 to 500:1, as determined by ordinary analytical procedures such as titration with acid.

In the processes of the invention there is first prepared a silica sol containing dense, nonagglomerated silica particles having an average diameter of from 10 to 130 millimicrons as observed by the electron microscope, the sol having a relative viscosity, as measured at 10% S102 and pH 10.0, of 1.15 to 1.55, and a specific conductance less than 4x10- mho/cm. at 28 C. and 10% SiOz. To this sol there is then added an alkali metal hydroxide, such as sodium, potassium, or lithium hydroxide. The quantity of hydroxide added is'enough to give a silica alkali oxide mole ratio in the sol of from 130:1 to 500:1.

It has been found that the aqueous silica sols produced as just described are stable against gelation. At ordinary temperatures of storage they are stable indefinitely, and even at temperatures as high as 95 they are stable for extended periods. They have improved compatibility with alcohols such as'ethanol, isopropanol, and ethylene glycol, ketones such as acetone, ethers such as butyl Carbitol, amines such as ethanolamine, and other organic liquids. The sols containing a relatively small proportion of an organic liquid such as an alcohol are resistant to freezing at low temperatures, but even if the sols are frozen, the silica is readily dispersible in the liquid medium after thawing.

The aqueous sols may be concentrated to a very high silica content merely by boiling off water. Sols which are stable against gelation for extended periods of time may be readily prepared containing silica in proportions as high as 50% by weight or more. These concentrated sols have a surprisingly low viscosity, and when the particles are in the smaller size range the turbidity may be not greater than a faint opal- ,escence. As compared with silica sols heretofore available at high concentration, the sols of this invention have substantially, improve film forming characteristics.

PROPERTIES or THE SOL-TREATED The aqueous silica sols which are amenable to treatment according to a process of this invention have certain well-defined physical and chemical properties. These properties are defined in terms of the size, shape, and density of the silica particles and their freedom from agglomeration, and the relative viscosity and specific conductance of the sols, as will now be fully described.

To be suitable for use in a process of this as above-described.

invention a sol must contain silica in the form of dense, non-agglomerated particles having an average diameter of from 10 to 130 millimicrons. In a preferred embodiment of the invention the average particle size is in the range from 13 to 60 millimicrons, and in a specific embodiment, giving especially advantageous results, the average particle size is from 15 to 30.

The size of the silica particles and the fact as to whether or not they are non-agglomerated, that is-substantially discrete, can be directly observed by means of an electron microscope. Since the limit of resolution of the electron microscope is well below the 10 millimicron limit of the particles, there is no difficulty is ascertaining whether particles of the desired range are present. On the other hand, sols prepared according to many conventional methods contain substantially no particles of the desired size range but contain aggregates of much smaller ultimate particles, and such aggregates should not be confused with the discrete particles here in referred to. Ordinarily, when a substantial proportion of aggregates of such under-size particles are present the relative viscosity is too high, and the lack of suitability will thus be indicated by the relative viscosity, determination referred to hereinafter.

In determining the size of silica particles by the electron microscope there is some densification efiected in the preparation of the sample. This is minimized by drying the sample at room temperature, under vacuum. Thus, the particle size limitation of 10 to 130 millimicrons referred to in the description of this invention is the size of particles as observed on an electron microscope on a sample originally containing 0.25% SiOz in Water and dried at C. under vacuum. The method of counting and measuring particles is described by Schaefier et al., in J ourn. Phys. and

Colloid Chem., 54, pp. 227-239 (February 1950).

The size uniformity of the silica particles in a sol may be determined by direct measurement and count on an electron micrograph prepared In a particular, preferred sol product of this invention the silica particles are relatively uniform in size, and since the size uniformity of the particles does not change substantially during the operation of a'process of the invention, it is necessary in making this product to start with a sol of relatively uniform particles. The uniformity should be such that at least 80 per cent of the particles have an average diameter of from 0.5 to 1.7 times the arithmetic mean particle diameter as determined from electron micrographs. If, for example, the diameter of the mean particle is millimicrons, 80 per cent of the mass of silica present will be in the form of particles having diameters in the range from 30 to 102 millimicrons.

The shape of the silica particles in a sol to be treated should be substantially spherical, as observed on an electron micrograph. Also, if the particles are substantially spherical the sol will conform to the Einstein viscosity relationship for spheroidal particles. The presence of an undue proportion of rod-shaped or other non-spheroidal shapes or forms of particles will cause the relative viscosity to be higher than the maximum permissible.

The density of the silica particles present may be determined by comparing their surface areas as calculated from electron micrographs with the surface areas as determined by nitrogen adsorption. On a suitably prepared electron microawn-res graphit'is-possible to measure the particle diameters'and'f'rom this measurement the specific surface area-that is, the surface areaper gramoi silica, can be' calculated on the assumption that theparticlesare substantially spherical and the spheres have a smooth external surface. Independently, the specific surface area may be determined'by nitrogen adsorption. Such a method is described in a new method for measuring the surface areas of finely divided materials and for determining the size of particles by P. H. Emmett in Symposium on New Methods for Particle :Size Determination in the Subsieve Range in the-Washington Spring Meeting of A. S. T. M., March l, 1941. i

When evaporating a sol to dryness for nitrogen adsorption determinations on the particles therein," the pH is important because if the sol is evaporated in the basic pH range the apparent nitrogen adsorption will be significantly less than on particles from a sol evaporated in the acidic pH'range of, say, 3 to 5. Consequently; in preparing the silica for surface area determinations, the sol should first be adjusted to a pH ofabout 3.5, and the water should be allowed to evaporate at a'temperature of 100 C. or below.

If the specific surface area as determined by nitrogen adsorption is not substantially greater, for examplenot more than 25% greater, than the specific surface area as calculated from electron'micrographs the"particles are dense and the sol is'suitable for use in a process of this invention. If the silica particles are'porous they willbe penetrated bynitrogen and the nitrogen adsorption will be relatively high, and consequently the specific surface area by nitrogen adsorption will also behigh-much higher than would be expected on the basis of the direct obseryation of the particles by means of the electron microscope. On the other hand, if the specificfsurface area by nitrogen adsorption is not substantially greater than thatcalculated from viscosity of a solution to the viscosity of th'esolvent. In the present instance the solution is the silica sol and the solvent is water which optionally may contain an organic liquid. The viscosity is measured at 25 C. on a sol having'a pH .of..l0

, mho/cm.

and containing 10% SiOz by weight. *A sol which is electrolyte-free must be adjusted to pH lO for the: purpose of this measurement by the addition of sodiurnhydroxide. A sol containing less than.10% SiOz may be concentrated up to 10% by vacuum evaporation of water at room temperature and a moreconc'entrated sol maybe diluted back with water to 10% for measurement.

The viscosity measurements used in determining relative viscosity may be made according to conventional methods provided thy'are capable of adequate precision. Measurements made with a capillary pipette, for instance, under properly controlled conditions of temperature are adequate for the purpose. I

The relative viscosity of asol treated according to a process of this invention should be 1.15 to 1.55 measured at 10% S102. Sols-having'a.

6 lower-"viscosity contain the silica in the former low-molecular weight polymers which are susceptible 'to further polymerization during the course of the process or thereafter, with-result- When the sol product to be produced by a process of this invention is to contain more than about 40% SiOz by weight and have maximum stability against gelation, it is preferred that the relative viscosity of 'thesol to which alkali is added be in the range of 1.15 to 1.30' as measured at 10 SiO2, because under these circumstances it is particularly desirable to avoid the presence of gel-like ca non-spheroidal particles.

The ionic content-of a sol to be treated according tothis invention must be quite low. Ideally, the ionic content should consist of only traces of either cations-or anions, but as a practical matter, somewhat larger amounts can be tolerated as hereinafter described.

The specific conductance of a silica sol which is to be treated according. to this invention may be determined in accordance with conventional practices, such as that described by Glasston'e, Text-Book of Physical Chemistry, at page 8'74 et. seq. "The specific conductance is measured at 28 Gron a sol-containinglO per cent by weight of silica expressed as $102. More concentrated sols maybe diluted. with distilled water for measurement. The specific conductivity ofa suitable sol should not exceed about 4X10 When'it is intended that the-final solproduct to be produced according to the invention shall contain more than about 40% SiO'z' by Weight and yet have maximum stability, it is preferred that the sol .to which alkali is added have a specific conductivity considerably less than 4X 10* mho/cm. as measured at 10%. SiOz; .and should not exceed about '1 10 mho./cm.' If 'it is desiredto concentrate'the final product above about ,SiO2 the specific conductance should not exceedabout6x15 mho/cm. 1

Having set up criteria by which a suitable sol may be judged methodswill now be described bywhich' silica sols may be prepared for treatment according to a process of the present invention. I

PREPARATION or THE SoL To BE TREATED sulfide, subjecting a sodium silicate solution to electro-osmosis'or electrolyzing a sodium silicate solution with a lead anode and mercury cathode. A particularly preferred method of making a'low' molecular weight sol is that of the Bird Patent 2,244,325;

According to the Bird patent an alkali metal silicate solution such as sodium silicate is passed through an ion-exchange material which removes most of the metal ions and gives a silica sol having a high ratio of silicate sodium; :..A1-'- ternatively, all the metal ions may be removed, in which case the sol may then be adjusted to a desired ratio by adding a requisite amount of sodium silicate or sodium hydroxide solution, taking care not to permit the sol to remain long in the pH range of 5 to '7 since in this range it is relatively unstable. It will be observed that the efliuent from the ionexchanger is a sol in which the silica is of low molecular weight.

, The low molecular Weight silica in any of these sols just described'may be condensed to form dense, discrete particles greater than 10 millimicrons in diameter by recently discovered techniques such as are described in application Ser. No. 65,536, filed December 15, 1948, by Bechtold and Snyder. Silica sols of dense particles which are preferred for use according to the present invention may be made according to such processes by heating a silica sol, prepared by ion exchange in the manner described in Bird 2,244,325 and stabilized with a small amount of alkali, to a temperature above 60 C. and adding further quantities of the same type of sol until at least five times as much silica has been added to the original quantity as was at first present. The product thus produced is stable against gelation at the pH of the present processes and it contains non-agglomerated silica particles having a molecular weight, as determined -by light scattering, of more than one- I halfmillion. The particle sizes are in excess of about 10 millimicrons and range upwardly to about 130 millimicrons. The particles in a particular sol are surprisingly uniform in size, but the size can be varied depending upon the process conditions under which they are formed. Thesols'have a relative viscosity as measured at 10% SiOz and pH 10, of 1.15 to 1.55.

The content of ions in a so] prepared from commercial materials by a process such as just described is sufiicient to make the specific conductance of the sol greater than 4x l0 mho/crn. at 28 C. and 10% SiOz, and hence the-ion con tent requires adjustment before treatment of the sol according to' a process of the present invention. The sols have a silica:alkali oxide mole ratio of from 60:1 to 130:1. It is necessary to decrease the cation content, and While this may be done in various ways, such as dialysis on the built-up sol, it is especially preferred to decrease the cation content by passing the sol through a cation exchange resin in the hydrogen form. Any insoluble cation-exchanger may beused for this purpose, the resins of sulfonated carbonaceous exchangersor of sulfonated or sulfited insoluble phenolformaldehyde resins, or acid-treated humic material, or other similar exchangers, being typical. Sulfonated coal, lignin, peat, or other insoluble sulfonated humic organic material may be used. .Even more preferable are the insoluble resins made from phenols, such as those made from phenol itself, diphenylol sulfone, catechol, or naturally occurring phenols, as found, for example, in quebracho, and an aldehyde, particularly formaldehyde, which are modified by the introduction of sulfonic groups either in the ring or on methylene groups. Cation-exchangers which -are stable in their hydrogen form are available commercially under such trade names as Amberlite, Ionex, Zeokarb, Nalcite, Ionac, etc.

The exchanger should be initially in the acid form. It will be understood that to regenerate aspent exchangerto the acid form even moderately weak acids will often be sufiicient' particularly if the acidity is derived from carboxylic acids or even phenolic groups. r

The exchanger is generally prepared in a granular form which is readily leached free of soluble acids or salts. If the exchanger is exhausted by use it may, readily be converted to the acid form by washing with a solution of an acid such as hydrochloric, sulfuric, formic, sulfamic, carboxylic, or the like.

One of the preferred cation-exchange resins for use according to the present invention is an aromatic hydrocarbon polymer containing nuclear sulfonic acid groups which is designated Dowex 50 and of the general type described in DAlelio 2,366,007 and which is fully described as to its characteristics, properties, and general mode of use in the Journal of the American Chemical Society for. November 1947, volume '69. No. 11, beginning at page 2830. I I

For convenience of reference suitable silica sols from which cations have been substantially completely removed by ion exchange as just described may be referred to as half-cycle deionized sols.,

Sols prepared according to the disclosure of Bechtold and Snyder application above .mentioned may contain varying amounts of anions such as sulfate, chloride, carbonate and bicarbonate, or the like. By carefully controlling the processes to avoid contamination by anions,.the anion concentration, and hence, thespecific conductance, may be held low enough so that the sol may be treated according to a process of the present invention without further adjustment of the anion content. Using deionized water for making up all solutions and carefully avoiding acid contamination of the sol from the regeneration of the cation exchange resin are expedients which may be followed to keep down the anion content. However, in some circumstances it is not practicable to observe these precautions and it is accordingly preferred to reduce the anion content if required by passing the sol through an anion exchanger in. the basic form.

Anion exchangers are generally well known, and the composition of the anion exchangers and their mode of use are fully described in the literature. Suitable materials are mentioned, for instance, in U. S. Patents 2,438,230 and 2,422,054. A description of both cationand anion-exchangers will be found in the May 1945 issue of Chemical Industries in an article entitled Ion-exchange by Sidney Sussman and Albert B. Mindler at pages 789 et seq.

While any of the anion-exchangers described will be found satisfactory, the insoluble resins obtained by the reaction of formaldehyde with an aromatic amine are particlarly useful. Such products are described, for instance, in theU. S. Patent 2,151,883 of Adams and Holmes. Reference is made, for instance, to the metaphenylenediamineformaldehyde type of anion-changer in the Ryznar Patent 2,438,230 stated above. A guanidine-type anion-exchanger may also be used.

As with the cation-exchangers, the technique of use is generally well understood and the anion-exchangers may be used in the mamiers customary in the art. It will be understood again that a considerable excess of anion-exchanger will ordinarily be used and the sulfate and chloride content will be lowered to an exceedingly wv u tivity measurement. the addition of ethyl or isopropyl alcohol, and

conductance be not greater than where R is the SiOz I alkali oxide mole ratio.

The sols may contain a water-soluble organic liquid as above described. The proportion of organic liquid may advantageously be from 1 to 50% by weight, based on the silica, from 5 to being preferred.

The sols are useful for all purposes where a silica intimately dispersed in a liquid continuous phase is desired. They are effective for treating textiles and textile fibers such as cotton, rayon, and wool. They can be used for treating paper for such purposes as increasing the stifiness or increasing the contrast of photocopying papers. They may be employed advantageously 6) X 10' mho/cm.

for plumping tanned leather such as chrometanned leather. K

By reason of their excellent organic comsilica sols containing larger amounts of electrolyte, they have improved film-forming characteristics and hence are particularly adapted for uses wherein film-forming ability is desired.

EXAMPLES The invention will be better understood by reference to the following illustrative examples:

Example 1 The sol utilized in this example was prepared from a commercially available silica sol containing relatively large amounts of electrolytes as indicated by both chemical analysis and conduc- This material gelled upon TABLE I 7 Analysis of purified sol SiOz 28.73% Na 0.03% S04 0.005% '01 l Not detectable Sulfated non-siliceous ash 0.12%

This purified sol was more completely characterized by its following properties:

TABLE II Properties of purified col 9 pH 3.7 Specific conductance (measured at 10% S102) 6 10- mho/cm. Relative viscosity (SiO2=10% T= C.; pH=10) 1.20 I Density (T=25 C.) 1.185 g./cc.-' I Turbidity (1) 67% Da (2) 16 millimicrons- Sn 181-m. /g. l.

Compatibility:. Excellent with acetone, ethanol,

morpholine, pyridine, and dioxane.

Turbidity is defined as percentage transmission as measured in a Beckmann spectrophotometer, model DU, segoat 400 millimicrons utilizing an absorption path of 1. cm.

D11 is defined as the average particle diameter in millimicrons based on the random measurement of more than 300 particles from an electron micrograph of the sol particles.

Sn is the actual surface area per gram, as measured by nitrogen adsorption, of dried powder obtained by allowing a portion of the sol to slowly evaporate at room temperature.

This purified sol was unstable to both heating and freezing but was modified according to processes of this invention to yield stable, useful products as follows: 7

The purified sol (1200 parts) was mixed with 58 parts of 1 N lithium hydroxide to give a sol having an SiOz L120 mol ratio of 200:1, and the resulting material concentrated until the silica content reached 40%. Then isopropyl alcohol (24 parts) was added, and the improved silica sol characterized. It exhibited excellent compatibility with acetone, alcohol, pyridine, and morpholine, and could be mixed with many water soluble organic polymers such as polyvinyl alcohol, gelatin and certain cellulose derivatives to form unique materials. The sol was resistant towards freezing and did not thicken or gel upon prolonged storage at elevated temperatures. The specific conductance for this material was 4.2 10- mho/cm., as measured at 28 C. and 10% SiOz.

Erample 2 A purified sol (890 parts) prepared as described in Example 1 was mixed with sodium hydroxide (17 parts of 1 Normal aqueous solution) to yield a sol having an SiO2 :NazO mole ratio of 500. The resulting sol had a pH of 8.2 and could be concentrated to a silica content of 44% by merely boiling off the necessary amount of water under rapid agitation. The resulting sol with its high silica content was fairly viscous at ordinary temperatures, but it became very fluid when warmed to about C. The concentrated sol was stable towards gelation after one months storage at C. and its viscosity did not change during several months storage at ordinary temperatures. The specific conductance of this sol as measured at 28 C. and a silica content of 10 per cent was 2.5x 10- mho/cm.

This highly concentrated silica sol was compatible with many water miscible organic solvents such as acetone, alcohol, ethylene glycol,

Another portion (890 parts) of a, purified sol, prepared as in Example 1 and characterized as in Table I and Table II of that example, was modified with enough sodium hydroxide (1 Normal aqueous solution) to change the pH of the sol to a value of 9.5. This corresponded to an SiOz NazO mole ratio of 150. The stabilized sol was then concentrated by boiling off water under rapid agitation until the silica content reached a value of 46 per cent. The resulting sol was relatively clear, with only a slight opalese cent appearance and was stable toward gelation after one months storage at 95 C. The specific conductance of this sol as measured at 28 C. and a silica content of 10 per cent was,4.8 l0- mho/cm. When the concentrated sol was diluted to a silica content of 10 per cent, its relative viscosity measured as prescribed was 1.20;

1'3 The properties of this sol were essentially the same as those of the sols previously described in Examples 1 and 2. ,Iclaim: 1, Ina process fol-producing a stable silica .501, the step comprising adding an alkali metal hy droxide to a silica sol containin amorpho silica particles which are dense, non-agglomerrated, spherical, and have anaverage diameter of to 130 millimicrons, the sol having a concentration of up to 50% by weight of SiOz, a relative viscosity, as measured at 10% SiOz and 121-1 10, or 1.15 to 1.55 and aspecificconductance; as measured at 10% S102 and 28C., of less than 4 1O- mho/cm., the amount of hydroxide added being enough to adjust the silica alkali. metal oxide mole ratio to from 130:1 to 500:1.

2. In a process for producing a stable silica sol, the step comprising adding lithiumhydroxide .to

a silica sol containing amorphous silica particles L which are dense, non-agglomerated, spherical, and have an average diameter .of 10v .to 130mi1limicronsthe sol having a concentration of up to 50% .by wei ht of S102, a, relative viscosity,- as

measured at 10% SiOz and pH 10, of 1.15 to 1.55

and a specific conductance, as measured at 10% Si02 and 28C., of less than 4X10- mho/cm., the amount of hydroxide added being enough to adjust the silica lithium oxide mole ratio to from 130:1 to 500:1. 1..

3..In.a process for producing a stablesilica sol, the step comprising adding sodium hydroxide to a silica sol containing amorphous silica particles which are dense; nonea'gglomerated, spherical, and have an average diameter of 10 to.130

millimicrons, the sol having a concentration of up. to. 50% by weightof S102, a relative viscosity, sis-measured at 10% SiO2 and pH 10, of.1.15.to 1.55 and a specific conductance, as measured at 10% S102 and 28 C., of less than 4x10 mho/cm., the amount. of hydroxide added being enough to adjust the silica: sodium oxide mole ratio to from 130:1 to 500:1. 1. In a process for producing a stable silica sol, thestep comprising adding an alkalimetal hydroxide to a silica solcontaining amorphous silica particles which-are dense, non-agglomerated, spherical, and have an average diameter of 10..to 130.millimicrons, the sol having-a r'elative viscosity, as measured at 10% S102 and pH-10, of 1.15 to 1.55 and a specificconductance, as

measured at 10% S102 and 28 C., of less" than 4x 10- v mho cnm, the amount ofhydroxide added being enough to adjust the silica: alkali metal oxide mole ratio to from 130:1 to 500:1, and concentrating the sol to a silica content of from to 50% by weight.

In aprocess for producinga stable'silica sol, the steps comprising adding an alkali metal hydroxide toa silica sol containing amorphous silica particles which are dense, non-agglomerated. spherical, and have and average diameter of 10 to 130 millimi'crons, the sol having a relative viscosity, as measured at 10% SiOz and pH 10, of 1.15- to 1.30 and a specific conductance, as measured at 10% SiOz and 28 C., of less than 1 X 10 mho /cm., the amount of hydroxideadded being-enough to adjust the silica: alkali metal oxide mole ratio to from 130:1 to 500 :1, and con-' centrating the sol to a silica contentof from 40to ated, spherical, and have anaverage diameter of 13 to 60 millimicrons, the sol having a relativeviscosity, as measured at 10% SiOz and. pH 10, of 1.15 to 1.30 and a specific conductance, as measured at 10% S102 and 28 0., of lessthan 1 x 10- mho/cm., the amount of hydroxide added being enough to adjust the silica: alkali metal oxide moleratio to from 130:1 to 500:1, and con centrating the sole toa silica content of from 40 to 50% by weight. 7. In a process for, producing a stable silica sol, the steps comprising adding an alkali metal hydroxide to a silica sol containing amorphous silica particles which are dense, non-agglomerated, spherical,-and have an average diameter of 10 to 130 millimicrons, the sol having a relative viscosity, as measured at 10% S102 and pH 10, of 1.15 to 1.30 and a specific conductance, as measured at 10% S102 and 28 C., of less than 6x 10- mho/cm., the amount of hydroxide added being enough to adjust the silica alkali 'metal oxide mole ratio to from 130:1 to 500:1, and concentrating the sol to a silica content of from to by weight.

8. In a process for producing a stable silica sol, the steps comprising adding an alkali metal hydroxide to a silica sol containing amorphous silica particles which are dense, non-agglomerated, spherical, and have an average diameter of 15 to 30 millimicrons, the sol having a relative viscosity, as measured at 10% S102 and pI-I'10, of 1.15 to 1.30 and a specific conductance, as measured at 10% S102 and 28 0., ofless than 6 1O -mho/cm., the amount of hydroxide added being enough to adjust the silica alkali metal oxide mole ratio to from 130:1 to 500:1, and concentrating the sol to a silica content of from 45 to 50 by weight.

9. In a process for producing a stable silica sol, the step, comprising adding an alkali metal hydroxide to a, silica sol containing amorphous silica particles which are dense, non-agglomerated, spherical, and have an average diameter of 10 to 130 millimicrons, and a uniformity of size such that at least of the particles havean average diameter of from 0.5 to 1.7 times'the arithmetic mean particle diameter, the sol hav his a. concentration of up to 50% by weight of 8102, a' relative viscosity, as measured at 10% SiOs andpH 10, 0f 1.'15 to 1.55 and a specific con 10. In a process for producing a stable silica sol, the. steps comprising treating 'witha cation exchanger and an anion exchanger a silica sol containing amorphous silica particles which are dense, non-agglomerated, spherical, and have an- SiOz and pH 10, of 1.15 to 1.55 and a specific:

conductance, as measured at 10% SiOzand 28 0., of more than 4X10 mho/cm., continuingthe ion exchange treatment until the specific conductance is less-than 4 l0 mho/cm., as measured at 10% SiOz and 28 C., and then adding an alkali metal hydroxide to the sol, the amount of hydroxide added being enough to adjust the silica: alkali metal oxide mole ratio to fr0rn130:1

to 500:1. V i

11; In a; processior producing a stable silica sol, the steps comprising adding any alkali imeta'll' hydroxide to a silica sol containing amorphous silica particles which are dense, non-agglomerated, spherical, and have an average diameter of 10 to 130 millimicrons, the sol having a concentration of up to 50% by weight of SiOz, a relative viscosity, as measured at 10% SiOz and pH 10, of 1.15 to 1.55 and a specific conductance, as measured at 10% S102 and 28 0., of less than 4 10- mho/cm., the amount of hydroxide added being enough to adjust the silica alkali metal oxide mole ratio to from 130:1 to 500:1, and to the resultant sol adding a water-soluble organic liquid.

' 12. An aqueous silica sol having a concentration of up to 50% by weight of S102, a silica alkali metal oxide mole ratio of from 130:1 to 500:1, a relative viscosity of from 1.15 to 1.55 as measured at 10% S102 and pH 10, and a specific conductance, as measured at 10% SiOz and 28 0., of less than where R is the silica alkali metal oxide mole ratio, and containing amorphous silica in the form of dense, non-agglomerated, spherical particles having an average particle diameter of 10 to 130 millimicrons.

13. An aqueous silica sol having a concentration of up to 50% by weight of S102, a silica lithium oxide mole ratio of from 130:1 to 500:1, a relative viscosity of from 1.15 to 1.55 as measured at 10% SlQz and pH 10, and a specific conductance, as measured at 10% SiOz and 28 C.,

of less than +30) X 10* mho/cm.

whereR is the silica lithium oxide mole ratio, and containing amorphous silica in the form of dense, non-agglomerated, spherical particles having an average particle diameter of to 130 millimicrons.

14. An aqueous silica sol having a concentration of up to 50% by weight of SiOz, a silica: sodium oxide mole ratio of from 130:1 to 500:1, a relative viscosity of from 1.15. to 1.55 as measured'at 10% S102 and pH 10, and a specific conductance, as measured at 10% SiOz and 28 0., of less than +30) X 10" mho/cm. I

where R is the silica alkali metal oxide mole ratio, and containing amorphous silica in the form of dense, non-agglomerated, spherical particles having an average particle diameter of 10 to 130 millimicrons, in a concentration of from t S102 by weight.

than Where R is the silica alkali metal oxide mole ratio, and containing amorphous silica in the form of dense, non-agglomerated, spherical particles having an average particle diameter of 10 to 130 millimicrons, the uniformity of size being such that at least 80% of the particles have an average diameter of from 0.5 to 1.7 times the arithmetic mean particle diameter.

17. An aqueous silica sol having a concentration of up to 50% by weight of SiO2, a silica alkali metal oxide mole ratio of from 150:1 to 300:1, a relative viscosity of from 1.15 to 1.55 as measured at 10% S102 and pH 10, and a specific conductance, as measured at 10% Si02 and 28 0., of less than +30) X 10' mho/cm.

where R is the silica alkali metal oxide mole ratio, and containing amorphous silica in the form of dense, non-agglomerated, spherical particles having an average particle diameter of 10 to millimicrons.

18. An aqueous silica sol having a silica alkali metal oxide mole ratio of from 130:1 to'500:1, a relative viscosity of from 1.15 to 1.30 as measured at 10% S102 and pH 10, and a specific conductance, as measured at 10% SiOz and 28 C., of less than +30) X 10- mho/cm.

where R'is the silica alkali metal oxide mole ratio, and containing amorphous silica in the form of dense, non-agglomerated, spherical particles having an average particle diameter of 15 to 30 mlllimicrons, in a concentration of from 45 to 50% S102 by weight.

20. A compositioncomprising a water-soluble organic liquid and an aqueous silica sol having a concentration of up to 50% by weight of S102, a silica: alkali metal oxide mole ratio of from 1'30:1 to 500:1, a-relative viscosity of from 1.15 to 1.55 as measured at 10% S102 and pH 10, and a specific conductance, as measured at 10% S102 and 28 C., of less than +6 X 10- mho/cin.

+6) X l0- mho/cm'.

ratio, and containing amorphous silica inthe' 17 form of dense, non-agglomerated, spherical particles having an average particle diameter of 10 to 130 millimicrons.

21. A composition comprising a water-soluble alcohol and an aqueous silica sol having a concentration of up to 50% by weight of $102, a silica alkali metal oxide mole ratio of from 130:1 to 500:1, a relative viscosity of from 1.15 to 1.55 as measured at 10% S102 and pH 10, and a specific conductance, as measured at 10% S102 and 28 C., of less than +30) X 10 niho/cm.

22. A composition comprising a water-soluble I alcohol and an aqueous silica sol having a silica alkali metal oxide mole ratio of from 130:1 to 500:1, a relative viscosity of from 1.15 to 1.55 as measured at 10% S102 and pH 10, and a specific conductance, as measured at 10% SiO2 and 28 C., of less than 30) X 1.0- mho/cm.

No references cited.

Certificate of Correction Patent No. 2,577,485 December 4, 1951 JOSEPH M. RULE It is hereby certified that error appears in the fprinted specification of the above numbered patent requiring correction as ollows:

Column 4, line 15, for difficulty is read dz'ficulty in; column 6, line 47, for 6X15 read 6X10; column 7, line 70, for form read fomns; column 14, line 9, for sole read sol and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 25th day of March, A. D 1952. V

THOMAS F. MURPHY,

Assistant Oommz'aaz'oner of Patents. 

1. IN A PROCESS FOR PRODUCING A STABLE SILICA SOL, THE STEP COMPRISIDNG ADDING AN ALKALI METAL HYDROXIDE TO A SILICA SOL CONTAINING AMORPHOUS SILICA PARTICLES WHICH ARE DENSE, NON-AGGLOMERRATED, SPHERICAL, AND HAVE AN AVERAGE DIAMETER OF 10 TO 130 MILLIMICRONS, THE SOL HAVING A CONCENTRATION OF UP TO 50% BY WEIGHT OF SIO2, A RELATIVE VISCOSITY, AS MEASURED AT 10% SIO2 AND PH 10, OF 1.15 TO 1.55 AND A SPECIFIC CONDUCTANCE, AS MEASURED AT 10% SIO2 AND 28* C., OF LESS THAN 4X10-4 MHO/CM., THE AMOUNT OF HYDROXIDE ADDED BEING ENOUGH TO ADJUST THE SILICA: ALKALI METAL OXIDE MOLE RATIO TO FROM 130:1 TO 500:1.
 12. AN AQUEOUS SILICA SOL HAVING A CONCENTRATION OF UP TO 50% BY WEIGHT OF SIO2, A SILICA: ALKALI METAL OXIDE MOLE RATIO OF FROM 130:1 TO 500:1, A RELATIVE VISCOSITY OF FROM 1.15 TO 1.55 AS MEASURED AT 10% SIO2 AND PH 10, AND A SPECIFIC CONDUCTANCE, AS MEASURED AT 10% SIO2 AND 28* C., OF LESS THAN 